Marine Resources Assessment for the Marianas Operating ... - SPREP

AUGUST 2005 FINAL REPORT
Morphometric analyses of the inner ear of right whales resulted in an estimated hearing frequency
range of approximately 10 Hz to 22 kHz, based on established marine mammal models (Parks et al.
2004). Research by Nowacek et al. (2004) on North Atlantic right whales suggests that received
sound levels of only 133 to 148 dB re 1 µPa-m for the duration of the sound exposure are likely to
disrupt feeding behavior; the authors did note, however, that a return to normal behavior within
minutes of when the source is turned off would be expected.
♦ Humpback Whale (Megaptera novaeangliae)
Description—Humpback whale adults are 11 to 16 m in length and are more robust than other
rorquals. The body is black or dark gray, with very long (about one-third of the body length) flippers
that are usually white (Jefferson et al. 1993; Clapham and Mead 1999). The head is larger than in
other rorquals. The flukes have a concave, serrated trailing edge; the ventral side is variably
patterned in black and white. Individual humpback whales may be identified using these patterns
(Katona et al. 1979).
Status—Humpback whales are classified as endangered under the ESA. There is no designated
critical habitat for this species in the North Pacific.
Recent information from photo-identification studies and genetic work suggests that there are
probably three stocks or populations in the North Pacific: Eastern, Central, and Western North Pacific
stocks (Baker et al. 1998; Calambokidis et al. 2001; Carretta et al. 2004). Calambokidis et al. (2001)
further suggested that up to six subpopulations of humpback whales in the North Pacific Ocean might
be recognized. The Western North Pacific stock is the stock considered to occur in the Marianas
study area (Jefferson personal communication); it is classified as a strategic stock by NOAA Fisheries
(Angliss and Lodge 2004). The Western North Pacific stock of humpback whales is clearly the
smallest of the various North Pacific stocks. Using mark-recapture analysis of photo-identification
data, Calambokidis et al. (1997) derived an estimate of 400 whales for the Western North Pacific
stock in 1993, although this estimate is probably biased downwards. The minimum population
estimate for the Western North Pacific stock of humpback whales is considered to be 367 individuals
(Angliss and Lodge 2004).
The Central North Pacific stock, which is the one that is best studied, overwinters in waters of the
main Hawaiian Islands and summers in British Columbia and Alaskan waters (Baker et al. 1986;
Calambokidis et al. 2001). Site fidelity to feeding areas is very high, but there is at least occasional
interchange among different breeding areas (Salden et al. 1999; Calambokidis et al. 2001). Most
individuals that breed in Hawai’i travel to waters of Alaska to feed (Calambokidis et al. 2001). The
Hawaiian breeding group shares some genetic inter-relatedness with the group that breeds off the
coast of Mexico (Baker et al. 1998) and even off Japan (Salden et al. 1999; Darling and Mori 1993),
but it is rare enough that these are still considered to be separate management stocks.
Habitat Preferences—Although humpback whales typically travel over deep, oceanic waters during
migration, their feeding and breeding habitats are mostly in shallow, coastal waters over continental
shelves (Clapham and Mead 1999). Shallow banks or ledges with high sea-floor relief characterize
feeding grounds (Payne et al. 1990; Hamazaki 2002). The habitat requirements of wintering
humpbacks appear to be determined by the conditions necessary for calving. Breeding grounds are in
tropical or subtropical waters, generally with shelter created by islands or reefs. Optimal calving
conditions are warm water (24° to 28°C) and relatively shallow, low-relief ocean bottom in protected
areas (behind reefs) apparently to take advantage of calm seas, to minimize the possibility of
predation by sharks, or to avoid harassment by males (Smultea 1994; Craig and Herman 2000;
Clapham 2000). Females with calves occur in significantly shallower waters than other groups of
whales, and breeding adults use deeper, more offshore waters (Smultea 1994; Ersts and Rosenbaum
2003).
Distribution—Humpback whales are globally distributed in all major oceans and most seas. They
generally are found during the summer on high-latitude feeding grounds and during the winter in the
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